Lenses designed for 35 mm single lens reflex cameras (“SLR lenses”) are well known in the art, and millions of such lenses have been manufactured since the late 1950's. An important feature of these lenses is that they must have a working distance that is sufficiently large to allow a reflex mirror to swing into the optical path in order to direct the image to a viewfinder system.
Optical attachments for increasing the focal length of SLR lenses are also well known, and are commonly called “teleconverters.” Teleconverters are an optical construction with negative optical power which is placed on the image side of an objective lens in order to increase the focal length of the objective lens. Teleconverters are almost always designed to maintain a sufficient working distance to be used with single lens reflex cameras. Unfortunately, teleconverters have several undesirable side effects, including: 1) they increase the focal ratio of the objective lens, thus reducing the “speed” of the objective plus teleconverter system; and 2) they magnify the aberrations of the objective lens, thus reducing the optical performance of the system.
Optical attachments for decreasing the focal length of an objective lens are also known in the art. These include: 1) afocal attachments placed on the object side of an objective; 2) re-imaging systems placed behind the image plane of an objective lens; and 3) a positive powered lens group placed between the objective and the image. The present disclosure is of the third type. Focal reducers of this type have several potential advantages, including 1) the aperture ratio is reduced; 2) the field of view of the objective lens can be largely maintained when used on a smaller format; 3) the vertex length of the objective-attachment system can be smaller than that of the objective lens alone; 4) the size and weight of a rear attachment can be much less than that of an afocal front attachment; 5) the chief ray angle of the objective-attachment system can be significantly reduced compared to that of the objective alone; and 6) the aberrations of the objective lens can be de-magnified, thus increasing resolution and contrast of the final image.
Despite these potential advantages, rear attachments for reducing the focal length of photographic objectives are notably uncommon due to a number of design and implementation challenges. Chief among these challenges is the fact that rear focal reducers of fairly simple construction tend to reduce the working distance of the system to a remarkable degree, thus making them unsuitable for use with single lens reflex cameras. An elaborate reverse-telephoto construction for eliminating this defect is disclosed in U.S. Pat. No. 5,499,069, but this solution is very complex and is not suitable for large apertures (e.g., small aperture ratios).
Another problem with rear focal reducers is that they inherently suffer from large undercorrected field curvature. This is dramatically shown by the focal reducers disclosed in U.S. Pat. Nos. 2,186,605 and 4,264,151. The focal reducers disclosed in U.S. Pat. No. 4,264,151 also suffer from a very large amount of barrel distortion. More recent examples, such as those disclosed in U.S. Pat. Nos. 4,634,235, 4,830,474 and 6,373,638 are reasonably well corrected for field curvature, but they suffer from large amounts of spherical aberration when used at a large aperture.
Recent advances in electronic viewfinders have resulted in a new type of interchangeable lens camera that has no need for a reflex mirror. Such cameras are commonly called “mirrorless cameras”, and they typically have a very short lens flange to image plane distance compared with SLR cameras with a similar image size. Examples of mirrorless cameras include the Micro Four Thirds cameras manufactured by Olympus and Panasonic, the Sony NEX series cameras, and the Samsung NX series cameras. The flange distance—the distance from the lens flange to the image plane—in all of these cameras is relatively small.
In Micro Four Thirds cameras the flange distance is approximately 20 mm, and in Sony Nex cameras the flange distance is approximately 18 mm. By contrast, the flange distance in 35 mm SLR cameras with a Nikon F mount is 46.5 mm. The large difference between the flange distance of 35 mm SLR cameras and mirrorless cameras allows for the design and implementation of a wide range of adapters to mount 35 mm SLR lenses onto mirrorless cameras. However, none of the adapters developed to-date reduce the focal length of the attached 35 mm SLR lens.
Thus, there is a need for optical attachment for reducing the focal length of an objective lens wherein said optical attachment in combination with said objective lens has a very high image quality at large aperture over a large field of view. There is additionally a need for such an adaptor to be as compact as possible, to be readily manufacturable, and to allow a wide variety of SLR lenses to be adpated to various mirrorless cameras.